docs: def load_rsa_private_key_file(rsakeyfile, passphrase):
      # type: (str, str) ->
      #        cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey
      """Load an RSA Private key PEM file with passphrase if specified
      :param str rsakeyfile: RSA private key PEM file to load
      :param str passphrase: optional passphrase
      :rtype: cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey
      :return: RSAPrivateKey
      """
      keypath = os.path.expandvars(os.path.expanduser(rsakeyfile))
      with open(keypath, 'rb') as keyfile:
          return cryptography.hazmat.primitives.serialization.\
              load_pem_private_key(
                  keyfile.read(),
                  passphrase.encode('utf8') if passphrase is not None else None,
                  backend=cryptography.hazmat.backends.default_backend()
              )